1. Leakage location

1.1 Tube plate leakage

In the production operation, the leakage of waste heat fire-tube boiler often occurs at the position where the weld of the tube plate of the front smoke box fails. The waste heat fire-tube boiler is composed of a cylinder and a tube plate, and the tube plate and the fire tube are welded. In addition to the design pressure load [1], the welding position of the tubesheet is also subjected to the temperature load with obvious difference between the high temperature smoke of the front smoke box (about 1100℃) and the saturated water temperature in the drum (about 250℃). Therefore, the welding seam of the tubesheet and the fire tube is subjected to stress changes under both pressure and temperature conditions, and the maximum stress is generated at this position. When there is corrosion in the welding of tubesheet during operation, it will cause weld failure and leakage.

1.2 Fire tube leakage

The 1100℃ high temperature flue gas of the self-immolating sulfur furnace enters the smoke pipe of the lower pot shell, and after heat exchange with the water in the pot shell, the flue gas cools to 380℃, and then enters the conversion section of the sulfur acid making system. The water vapor mixture in the pot shell returns to the upper drum through the rising pipe, and then output saturated steam after separation in the drum.

If the boiler feed water and pot water do not meet the requirements of GB/T 12145(steam quality of thermal generating units and steam power equipment), It will cause a large number of silicon-containing compounds to enter the drum, and colloidal silicon will decompose into active silicon in the environment of high temperature, high pressure and high pH value, which will increase the content of active silicon in the pot water, and form adhesive scale and deposit on the drum, down tube, shell wall and fire tube [2]. Since the fire tube behind the front smoke box tube plate has the largest temperature difference, if the furnace water does not meet the requirements, a large amount of scale will accumulate in the fire tube behind the ceramic insulation tube behind the front smoke box tube plate. With the accumulation of scale, the heat transfer efficiency of the fire tube at this position will decrease. The material of the fire tube is 20G, allowing the metal wall temperature to be 430℃. The temperature of the metal wall is 280℃, and the temperature difference between the metal wall and the pot water is about 30℃. However, when the scale in the pot and the smoke pipe is accumulated, the heat transfer is seriously affected. When the scale is 1mm, the temperature difference reaches about 200℃; when the scale is 3mm, the temperature of the metal wall rises to 580℃, far exceeding the allowable use temperature of the material. The fire tube will bulge, deform or even crack under internal pressure at any time, resulting in fire tube leakage [2].

2. Leakage phenomenon

1. The boiler leakage will increase the boiler water supply and reduce the operating pressure.

2. Boiler leakage (under the condition that the heat secondary valve is open at the same degree) will lead to a lower temperature of the inlet furnace.

3. The leakage of water vapor from the boiler into the converter will absorb SO3 and cause the temperature to rise, and the temperature change in each section is abnormal, which is difficult to control, and in the process of gradually decreasing the temperature in the first section, the reaction will gradually move back to the second and third sections, resulting in high temperature in the second and third sections.

4. The leakage of boiler, economizer and high temperature superheater leads to the high temperature inlet flue gas temperature of the high temperature absorber.

5. The occurrence of a large number of leaks in the boiler and economizer will lead to a significant reduction in the dry acid absorption concentration (or a significant reduction in the amount of water added), and the bottom air chamber of the heat exchanger will produce more condensed acid.

6. Boiler leakage will lead to a significant increase in the water content of the inlet gas of the first stage of the converter [3].

3. Cause

1. In the process of repeatedly opening and stopping the device, thermal expansion and cold contraction occur, which makes the refractory material and thermal insulation steel pipe on the front tube plate fall off, and directly exposes the tube plate to high temperature flue gas, and is affected by condensing acid fog, resulting in sulfuric acid stress corrosion. Since this position is the weakest point of the boiler, stress concentration occurs and internal pressure is subjected to, resulting in tube plate weld failure and leakage.

2. Liquid sulfur is atomized from the sulfer gun into the sulfur burning furnace for combustion. If the atomization effect is not good, unburned sulfur vapor will be generated, and vulcanization corrosion will occur on the fire tube [4]. Sublimed sulfur and the fire tube produce iron sulfide under high temperature environment, and the pipe wall will be worn and thinned due to the long-term erosion of flue gas, and the outer wall of the fire tube will be subjected to boiler pressure, resulting in the fire tube internal concave damage and cracking and leakage.

3. During operation, in order to ensure the standard of phosphoric acid in the pan water, the dosage is greatly increased, and there are too many suspended substances and dissolved salts in the pan water, the surface viscosity of the pan water becomes larger, the resistance of bubbles rising increases, and a large number of tiny bubbles are stuck on the interface of soda water, resulting in “ Steam plug ” Resulting in overheating of the shell and the upper part of the tube plate, uneven heating of the tube plate, local overheating, resulting in a sudden increase in thermal stress, making the tube plate in front of the shell unstable deformation.

References

[1] YU Rui. Stress Analysis and Strength Check of hot pot boiler in 1800Kt/a sulfur acid making Unit [J]. Sulfuric Acid Industry, 2012 (03):43-47.

[2] Wen Zengyu. Analysis of tube plate leakage of sulfur-acid waste heat boiler [J]. Industrial Boiler, 2021 (04):47-51. (in Chinese)

[3] Zhang Weihong. Analysis and judgment of waste heat boiler leakage in sulfur acid production [J]. Sulfur and Phosphorus Design and Powder Engineering, 2015 (05):46-48. (in Chinese)

[4] Xie Jiankang. Analysis of pipe explosion causes and preventive measures of sulfur acid waste heat boiler [J]. Sulfur and Phosphorus Design and Powder Engineering, 2022 (06):30-34. (in Chinese)